Optical amplifying and relaying system

ABSTRACT

An optical amplifying and relaying system capable of easily and highly accurately monitor troubles in optical amplifiers provided in an up and a down optical fiber transmission line opposing each other is disclosed. Monitoring light signal folding-back lines including variable optical attenuators  4   a  and  4   b  and wavelength selective reflecting means  5   a  and  5   b , respectively, are provided between optical transmission lines L 1  and L 2,  which oppose each other and on which optical amplifiers  4   a  and  4   b  are disposed each other.

The present Application is a Divisional Application of U.S. patentapplication Ser. No.10/098,308 filed on Mar. 18, 2002.

This application claims benefit of Japanese Patent Application No.2001-078308 filed on Mar. 19, 2001, the contents of which areincorporated by the reference.

BACKGROUND OF THE INVENTION

The present invention relates to optical amplifying and relaying systemsand, more particularly, to optical amplifying and relaying systems foramplifying light signals in optical fiber transmission lines or the likeand also monitoring the transmission line state.

The optical communication utilizing optical fibers has various meritscompared to conventional electric communication with electric signalsthrough the copper wire. Particularly, the optical communication can beadopted for high rate and large capacity communication and is excellentin anti-noise characteristics against electromagnetic noise or the like.In the optical communication, coherent light beams such as laser beamsare transmitted on very thin optical fiber cables. Although the opticalfiber cable has low optical attenuation characteristic and permits longdistance transmission, in order to maintain the signal quality the lightsignal is amplified by optical amplifier/relays provided at apredetermined interval. Prior art techniques in such technical field aredisclosed in Japanese Patent Laid-Open No. 9-116502 entitled “HighOutput Optical Amplifier/Relay having monitoring Loop-Back Circuit”,Japanese Patent Laid-open No. 9-153862 entitled “Monitoring Method inOptical Amplifying/Relaying Transmission System” and Japanese PatentLaid-Open No. 2000-59306 entitled “Optical Amplifier/Relay” and soforth.

FIG. 17 is a block diagram showing the structure of a prior art opticalamplifier/relay (or optical amplifying and relaying system). Thisoptical amplifier/relay 1 comprises a first and a second opticalamplifier 2 a and 2 b, a first (optical) coupler 3 a, a second coupler 3b, a first and a second wavelength selective reflecting means 5 a and 5b and a first and a second terminal part 6 a and 6 b. In this prior arttechnique, light branching/wavelength selective reflecting meansconnected at subsequent stages to the optical amplifiers 2 a and 2 b inpaired optical transmission lines for transmission and reception,transmit monitoring signals to the opposite lines.

FIG. 18 is a view illustrating operation of the optical amplifier/relayshown in FIG. 17 in the case of transmitting monitoring light signals tothe opposite optical transmission lines for the monitoring thereof. Inthis optical amplifier/relay, in the optical transmission line amonitoring light signal (λsv) and a main light signal (λ1 to λ4) areinputted to the optical amplifier 2 a and transmitted via subsequentstage optical branching function constituted by the first opticalcoupler 3 a, the wavelength selective reflecting means 5 a and thesecond coupler 3 b to the opposite optical transmission line, andmonitoring light signal(λsv) and the reflectivity of the optical gratingare obtained on the opposite optical transmission line side.

In this optical amplifier/relay, the light intensity variation in theoptical amplifier in each line and the light intensity variation of themonitoring light signal transmitted to the opposite line are equal. Inthis case, a problem arises in the event of a trouble occurred in theoptical amplifier, the output of which is subject to very littlevariation, that the trouble can not be recognized or that therecognition requires long time. Also, the monitoring light signal foldedback to the opposite line permits obtaining only data concerning theoutput level of the optical amplifier.

SUMMARY OF THE INVENTION

An object of the present invention, accordingly, is to provide a highaccuracy optical amplifying and relaying system capable of solving oralleviating the above problem inherent in the prior art.

According to a first aspect of the present invention, there is providedan optical amplifying and relaying system comprising an up and a downoptical transmission line opposing each other, amplifiers each providedon each of the optical transmission lines, and monitoring light signalfolding-back lines connected between the two optical transmission linesand each including an optical coupler for taking out a monitoring lightsignal led to the own optical transmission line and a wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the own optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein: theoptical amplifying and relaying system further comprises variableoptical attenuators each provided between each optical coupler and theassociated wavelength selective reflecting means.

The optical couplers are each provided on the optical transmission linein the output side of each optical amplifier. The optical couplers areeach provided on the optical transmission line on the input side of eachoptical amplifier. Wavelength selective reflecting means are providedtwo on opposites of each of the variable optical attenuators, the twowavelength selective reflecting means being operative to reflect lightsignals of different wavelengths for transmission to the oppositeoptical transmission line. The monitoring light signal folding-backlines each include a pair of lines for transmitting a light signal fromthe output side of the optical amplifier on the own optical transmissionline to the input side of the optical amplifier on the opposite opticaltransmission line. A light signal led to each monitoring light signalfolding-back line is transmitted via a separate optical coupler andwavelength selective reflecting means of different wavelengths to theopposite optical transmission line.

According to a second aspect of the present invention, there is providedan optical amplifying and relaying system comprising an up and a downoptical transmission line opposing each other, amplifiers each providedon each of the optical transmission lines, and monitoring light signalfolding-back lines connected between the two optical transmission linesand each including an optical coupler for taking out a monitoring lightsignal led to the own optical transmission line and a wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the own optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein variableoptical attenuators are each provided between each optical coupler andthe associated wavelength selective reflecting means, the monitoringlight signal branched and reflected by the optical coupler on eachoptical transmission line being transmitted via the optical coupler onthe opposite optical transmission line to the optical amplifier outputside thereof.

According to a third aspect of the present invention, there is providedan optical amplifying and relaying system comprising an up and a downoptical transmission line opposing each other, amplifiers each providedon each of the optical transmission lines, and monitoring light signalfolding-back lines connected between the two optical transmission linesand each including an optical coupler for taking out a monitoring lightsignal led to the own optical transmission line and a wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the own optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein variableoptical attenuators are each provided between each optical coupler andthe associated wavelength selective reflecting means, the monitoringlight signal folding-back lines are each provided on the input side ofthe optical amplifier on the own optical transmission line.

According to a fourth aspect of the present invention, there is providedan optical amplifying and relaying system comprising an up and a downoptical transmission line opposing each other, amplifiers each providedon each of the optical transmission lines, and monitoring light signalfolding-back lines connected between the two optical transmission linesand each including an optical coupler for taking out a monitoring lightsignal led to the own optical transmission line and a wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the own optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein variableoptical attenuators are each provided between each optical coupler andthe associated wavelength selective reflecting means, and the monitoringlight signal branched and reflected by each coupler on optical amplifieroutput side of the own optical transmission line is transmitted via theoptical coupler and optical amplifier input side on the opposite opticaltransmission line to the same opposite optical transmission line.

According to a fifth aspect of the present invention, there is providedan optical amplifying and relaying system comprising an up and a downoptical transmission line opposing each other, amplifiers each providedon each of the optical transmission lines, and monitoring light signalfolding-back lines connected between the two optical transmission linesand each including an optical coupler for taking out a monitoring lightsignal led to the own optical transmission line and a wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the own optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein variableoptical attenuators are each provided between each optical coupler andthe associated wavelength selective reflecting means, and opticalcouplers are provided on the output and input sides of each opticalamplifier, and the branched and reflected monitoring light signals oneach optical transmission line side are transmitted via the output andinput sides, respectively, of the optical amplifier on the oppositeoptical transmission line to the same opposite optical transmissionline.

According to a sixth aspect of the present invention, there is providedan optical amplifying and relaying system comprising an up and a downoptical transmission line opposing each other, amplifiers each providedon each of the optical transmission lines, and monitoring light signalfolding-back lines connected between the two optical transmission linesand each including an optical coupler for taking out a monitoring lightsignal led to the own optical transmission line and a wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the own optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein variableoptical attenuators are each provided between each optical coupler andthe associated wavelength selective reflecting means, and on eachoptical transmission line a monitoring light signal branched andreflected by an optical amplifier output side optical coupler via anoptical coupler and the optical amplifier input side on the oppositetransmission line to the same opposite optical transmission line, whilea monitoring light signal branched and reflected by an optical amplifieroutput side on the opposite optical transmission line to the sameopposite optical transmission line.

According to a seventh aspect of the present invention, there isprovided an optical amplifying and relaying system comprising an up anda down optical transmission line opposing each other, amplifiers eachprovided on each of the optical transmission lines, and monitoring lightsignal folding-back lines connected between the two optical transmissionlines and each including an optical coupler for taking out a monitoringlight signal led to the own optical transmission line and a wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the own optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein variableoptical attenuators are each provided between each optical coupler andthe associated wavelength selective reflecting means, and on eachoptical transmission line the light signal led to the monitoring lightsignal folding-back line is branched by a separate optical coupler, thenreflected by a plurality of wavelength selective reflecting means ofdifferent wavelengths and then transmitted to the opposite opticaltransmission line.

Wavelength selective reflecting means is provided preceding to andsubsequent to the variable optical attenuator. An optical isolator isprovided as an intermediate stage in each monitoring light signalfolding-back line, and each wavelength selective reflecting means usesan optical fiber grating.

Other objects and features will be clarified from the followingdescription with reference to attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a first embodiment ofthe optical amplifying and relaying system according to the presentinvention;

FIG. 2 is a view showing the structure of a second embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 3 is a view showing the structure of a third embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 4 is a view showing a fourth embodiment of the optical amplifyingand relaying system according to the present invention;

FIG. 5 is a view showing the structure of a fifth embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 6 is a view showing the structure of a sixth embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 7 is a view showing the structure of a seventh embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 8 is a view showing the structure of an eighth embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 9 is a view showing the structure of a ninth embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 10 is a view showing the structure of a tenth embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 11 is a view showing an eleventh embodiment of the opticalamplifying and relaying system according to the present invention;

FIG. 12 is a view showing the structure of a twelfth embodiment of theoptical amplifying and relaying system according to the presentinvention;

FIG. 13 is a view showing the structure of a thirteenth embodiment ofthe optical amplifying and relaying system according to the presentinvention;

FIG. 14 is a view showing the structure of a fourteenth embodiment ofthe optical amplifying and relaying system according to the presentinvention;

FIG. 15 shows the operation of transmission of the monitoring lightsignals in the optical amplifying and relaying system according to thepresent invention to the opposite lines;

FIG. 16 shows an example of the level characteristic of the monitoringlight signals folded back to the opposite lines in the opticalamplifying and relaying system according to the present invention;

FIG. 17 is a block diagram showing the structure of a prior art opticalamplifier/relay (or optical amplifying and relaying system); and

FIG. 18 is a view illustrating operation of the optical amplifier/relayshown in FIG. 17.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the drawings. For the sake of the brevity, elementscorresponding to those in the prior art described above are designatedby like reference numerals.

FIG. 1 is a block diagram showing the structure of a first embodiment ofthe optical amplifying and relaying system according to the presentinvention. The illustrated first embodiment of the optical amplifyingand relaying system 1A includes two (or a pair of) optical amplifiers,i.e., an up and down line optical amplifiers 2 a and 2 b correspondingto an up and a down optical fiber transmission lines L1 and L2,respectively. As stages subsequent to the two optical amplifiers 2 a and2 b are provided optical couplers (or optical branchers) 3 a and 3 b,variable optical attenuators 4 a and 4 b, wavelength selectivereflecting means 5 a and 5 b, terminal parts 6 a and 6 b and a fixedattenuator 7. In this specification, by the term “optical coupler” isgenerally referred to as an optical function element having a functionof branching and combining light. The fixed attenuator 7 is notessential.

The optical couplers 3 a and 3 b each branch a main and a monitoringoutput light signal. The variable optical attenuators 4 a and 4 b adjustthe light intensity levels of the light signals branched by the opticalcouplers 3 a and 3 b. The wavelength selective reflecting means 5 a and5 b transmit (pass) only the main light signals outputted from thevariable optical attenuators 4 a and 4 b and reflect only the monitoringlight signals therefrom. The monitoring light signals reflected by thewavelength selective reflecting means 5 a and 5 b pass through thevariable optical attenuators 4 a and 4 b again, thence pass through thefixed attenuator 7 and thence fed out via the output side opticalcouplers 3 b and 3 a and optical amplifiers 2 b and 2 a, respectively,to the opposite optical fiber transmission lines L2 and L1.

FIG. 15 shows the operation of transmission of the monitoring lightsignals (λsv) in the optical amplifying and relaying system 1A accordingto the present invention to the opposite lines. The optical coupler 3 abranches part of the main and monitoring light signals (λ1 to λ4) and(λsv) outputted from the optical amplifier 2 a from the opposite opticalfiber transmission line. The main and monitoring light signals (λ1 toλ4) and (λsv) branched by the optical coupler 3 a pass through thevariable optical attenuator 4 a and fed to the wavelength selectivereflecting means 5 a, which transmits only the main light signal (λ1 toλ4) and reflects only the monitoring light signal (λsv). The monitoringlight signal (λsv) reflected by the wavelength selective reflectingmeans 5 a passes through the variable optical attenuator 4 a again, andfed out by the optical coupler 3 b provided on the output side of theopposite optical amplifier 2 b to the opposite optical fibertransmission line. The attenuation level of the variable opticalattenuator 4 a (which prescribes the level of the monitoring lightsignal fed out to the opposite optical fiber transmission line) is apredetermined according to the state of the optical amplifier 2 a (forinstance, large when the number of troubles in an excited laser diode isincreased or when the optical amplifier input level is reduced), and itis possible to determine the state of the optical amplifying andrelaying system 1A in an optical amplifying and relaying system monitor(not shown) installed in a terminal station.

FIG. 16 shows an example of the level characteristic of the monitoringlight signals folded back to the opposite lines when setting theattenuation level of the variable optical attenuators 4 a and 4 b basedon the output state of the exciting laser diode LD in the opticalamplifying and relaying system 1A shown in FIG. 1. State A shows theoptical attenuation level and the monitoring light signal level of thevariable optical attenuator 4 a when the output level of the exciting LDis a rated level. In this state, the attenuation level of the monitoringlight signal is 0 dB, and the level thereto is an initial level. State Bshows the optical attenuation level and the monitoring light signallevel of the variable optical attenuator 4 a when the output level ofthe exciting LD is reduced to one half. In this state, the attenuationlevel of the monitoring light signal is −3 dB with respect to theinitial level. State C shows the optical attenuation level and themonitoring light signal level of the variable optical attenuator 4 awhen the output of the exciting LD is turned off. In this state, theattenuation level of the monitoring light signal is −6 dB with respectto the initial level. The state of the exciting LD in the opticalamplifying and relaying system 1A can be monitored by determining thedifferent light signal levels with the above terminal station monitor.

FIG. 2 is a view showing the structure of a second embodiment of theoptical amplifying and relaying system according to the presentinvention. The second embodiment of the optical amplifying and relayingsystem 1B comprises, in addition to all the elements in the firstembodiment shown in FIG. 1, wavelength selective reflecting means 5 aand 5 c, and 5 b and 5 d provided preceding to and succeeding to thevariable optical attenuators 4 a and 4 b, respectively. The two pairs ofwavelength selective reflecting means 5 a and 5 c, and 5 b and 5 d,reflect different wavelength monitoring light signals to be transmittedto the opposite lines L2 and L1. It is possible to make highly accuratemeasurement of the attenuation levels of the variable opticalattenuators by determining the levels of the transmitted monitoringlight signals of the two different wavelengths.

FIG. 3 is a view showing the structure of a third embodiment of theoptical amplifying and relaying system according to the presentinvention. This optical amplifying and relaying system 1C is the same instructure as the first embodiment of the optical amplifying and relayingsystem 1A shown in FIG. 1 except for that the monitoring light signalfolding-back lines are provided not as succeeding stage (i.e., on theoutput side) but on the input side of the up and down optical amplifiers2 a and 2 b.

FIG. 4 is a view showing a fourth embodiment of the optical amplifyingand relaying system according to the present invention. This opticalamplifying and relaying system 1D is a combination of the previoussecond and third embodiments of the optical amplifying and relayingsystems 1B and 1C. More specifically, the monitoring light signalfolding-back lines are provided in the input side of the up and downoptical amplifiers 2 a and 2 b, respectively, and the wavelengthselective reflecting means 5 a and 5 c and 5 b and 5 d are providedpreceding to and succeeding to the variable optical attenuators 4 a and4 b.

FIG. 5 is a view showing the structure of a fifth embodiment of theoptical amplifying and relaying system according to the presentinvention. This optical amplifying and relaying system 1E furthercomprises optical couplers 3 d and 3 c, which transmit monitoring lightsignals having been branched and reflected on the output side of theoptical amplifiers 2 a and 2 b via the opposite optical amplifiers 2 band 2 a to the opposite optical fiber transmission lines L2 and L1.

FIG. 6 is a view showing the structure of a sixth embodiment of theoptical amplifying and relaying system according to the presentinvention. This optical amplifying and relaying system 1F, like theoptical amplifying and relaying system 1E shown in FIG. 5, furthercomprises optical couplers 3 c and 3 d, which transmit monitoring lightsignals having been branched and reflected on the output side of theoptical amplifiers 2 a and 2 b via the opposite optical amplifiers 2 band 2 a to the opposite optical fiber transmission lines L2 and L1. Thisembodiment of the system 1F, like the FIG. 2 system, further compriseswavelength selective reflecting means 5 a and 5 c, and 5 b and 5 dprovided preceding to and succeeding to the variable optical attenuators4 a and 4 b.

FIG. 7 is a view showing the structure of a seventh embodiment of theoptical amplifying and relaying system according to the presentinvention. This optical amplifying and relaying system 1G comprisesoptical couplers 3 a and 3 c, and 3 b and 3 d provided on the output andinput sides of the optical amplifiers 2 a and 2 b, respectively. Thebranched and reflected monitoring light signals are transmitted not viaand via the opposite optical amplifiers 2 b and 2 a to the oppositeoptical fiber transmission lines L2 and L1.

FIG. 8 is a view showing the structure of an eighth embodiment of theoptical amplifying and relaying system according to the presentinvention. This optical amplifying and relaying system 1H, like theoptical amplifying and relaying system 1G shown in FIG. 7, comprisesoptical couplers 3 a to 3 d, which are provided on the output and inputsides of the optical amplifiers 2 a and 2 b and transmit the branchedand reflected monitoring light signals via the opposite opticalamplifiers 2 b and 2 a to the opposite optical fiber transmission linesL2 and L1. Also, like the FIG. 2 system, this embodiment of the system1H further comprises wavelength selective reflecting means 5 a and 5 e,5 b and 5 f, 5 a and 5 g, and 5 d and 5 h provided preceding to andsucceeding to the variable optical attenuators 4 a to 4 d.

FIG. 9 is a view showing the structure of a ninth embodiment of theoptical amplifying and relaying system according to the presentinvention. In this optical amplifying and relaying system 1I, themonitoring light signals branched and reflected by the optical couplers(or branchers) 3 a and 3 b on the output side of the optical amplifiers2 a and 2 b, are transmitted via optical couplers 3 d and 3 c to theoptical amplifiers 2 b and 2 a on the opposite optical fibertransmission lines L2 and L1. Also, the monitoring light signalsbranched and reflected by optical couplers 3 c and 3 d on the input sideof the optical amplifiers 2 a and 2 b are transmitted via the opticalcouplers 3 c and 3 d to the output side of the optical amplifiers 2 band 2 a on the opposite optical fiber transmission lines L2 and L1.

FIG. 10 is a view showing the structure of a tenth embodiment of theoptical amplifying and relaying system according to the presentinvention. In the optical amplifying and relaying system 1J, themonitoring light signals branched and reflected on the output side ofthe optical amplifiers 2 a and 2 b are transmitted via the oppositeoptical amplifiers 3 d, 3 c to the opposite optical fiber transmissionlines, and the monitoring light signals branched and reflected on theinput side of the optical amplifiers 2 b and 2 a are transmitted via theopposite optical amplifiers to the opposite optical fiber transmissionlines L2 and L1. Also, wavelength selective reflecting means 5 a and 5c, 5 b and 5 d, 5 e and 5 g, and 5 f and 5 h are provided preceding toand succeeding to the variable optical attenuators 4 a to 4 d.

FIG. 11 is a view showing an eleventh embodiment of the opticalamplifying and relaying system according to the present invention. Inthis optical amplifying and relaying system 1K, the light signals ledvia optical couplers (or branchers) 3 a and 3 b to the monitoring lightsignal folding-back lines, are taken out by separator optical couplers(or branchers) 3 c and 3 d and reflected by a plurality of wavelengthselective reflecting means 5 a and 5 b of different wavelengths to betransmitted to the opposite lines L2 and L1. This structure permitsmonitoring data of a plurality of optical amplifying and relayingsystems.

FIG. 12 is a view showing the structure of a twelfth embodiment of theoptical amplifying and relaying system according to the presentinvention. In this optical amplifying and relaying system 1L, like theFIG. 11 system, the main light signals led via the optical couplers 3 aand 3 b to the monitoring light signal folding-back lines are taken outby the optical couplers 3 c and 3 d and transmitted by the wavelengthselective reflecting means 5 a and 5 b of different wavelengths to theopposite lines L2 and L1. This structure thus permits monitoring data ofa plurality of optical amplifying and relaying systems. Furthermore,wavelength selective reflecting means 5 c and 5 d are provided precedingto the optical couplers (or branchers) 3 c and 3 d, thus permittinghighly accurate optical attenuation level monitoring.

FIG. 13 is a view showing the structure of a thirteenth embodiment ofthe optical amplifying and relaying system according to the presentinvention. In this optical amplifying and relaying system 1M, themonitoring light signals branched and reflected by optical couplers (orbranchers) 3 a and 3 b on the output side of optical amplifiers 2 a and2 b are transmitted by optical couplers 3 c and 3 d via opticalamplifiers 2 b and 2 a to the opposite optical fiber transmission linesL2 and L1. The system 1M also comprises wavelength selective reflectingmeans 5 a and 5 c, and 5 b to 5 d. Main light signals inputted to theoptical amplifiers 2 a and 2 b are branched by optical couplers (orbranchers) on the input side of the optical amplifiers 2 a and 2 b, andthe branched light signals are inputted to PDs (i.e., photo-diodes). Theoptical attenuation levels of the variable optical attenuators 4 a and 4b are controlled based on the output levels of the PDs.

FIG. 14 is a view showing the structure of a fourteenth embodiment ofthe optical amplifying and relaying system according to the presentinvention. In this optical amplifying and relaying system 1N, opticalisolators 8 a and 8 b are provided as intermediate stages in themonitoring light signal folding-back lines. The wavelength selectivereflecting means 5 a and 5 b adopt optical fiber gratings. The system 1Nfurther uses optical couplers 3 a to 3 d.

As has been described in the foregoing, with the optical amplifying andrelaying system according to the present invention the followingpractical pronounced effects are obtainable. More specifically, in anoptical transmission system comprising an up and a down optical fibertransmission line opposing each other, terminal stations installed atthe opposite terminals of these lines and a plurality of opticalamplifying and relaying systems cascade connected between the opticalfiber transmission lines, variable optical attenuators may be providedin monitoring light signal folding circuits installed in the individualoptical amplifiers for easily and highly accurately determining thelevels of the monitoring light signals folded back to the opposite linesin dependence on the state of the optical amplifying and relaying systemto be monitored. It is further possible to monitor a plurality of dataof the optical amplifying and relaying systems.

Changes in construction will occur to those skilled in the art andvarious apparently different modifications and embodiments may be madewithout departing from the scope of the present invention. The matterset forth in the foregoing description and accompanying drawings isoffered by way of illustration only. It is therefore intended that theforegoing description be regarded as illustrative rather than limiting.

1. An optical amplifying and relaying system comprising: an up and adown optical transmission line opposing each other; amplifiers eachprovided on each of the optical transmission lines; and monitoring lightsignal folding-back lines connected between the two optical transmissionlines and each including an optical coupler for taking out a monitoringlight signal led to one optical transmission line and wavelengthselective reflecting means for transmitting the monitoring light signalreceived from the one optical transmission line by folding-backtransmission to the opposite optical transmission line, wherein theoptical amplifying and relaying system further comprises variableoptical attenuators each provided between each optical coupler and theassociated wavelength selective reflecting means.
 2. The opticalamplifying and relaying system according to claim 1, wherein the opticalcouplers are each provided on the optical transmission line on the inputside of each optical amplifier.
 3. An optical amplifying and relayingsystem comprising: an up and a down optical transmission line opposingeach other; amplifiers each provided on each of the optical transmissionlines; and monitoring light signal folding-back lines connected betweenthe two optical transmission lines and each including an optical couplerfor taking out a monitoring light signal led to one optical transmissionline and wavelength selective reflecting means for transmitting themonitoring light signal received from the one optical transmission lineby folding-back transmission to the opposite optical transmission line,wherein variable optical attenuators are each provided between eachoptical coupler and the associated wavelength selective reflectingmeans, and the monitoring light signal folding-back lines are eachprovided on the input side of the optical amplifier on the one opticaltransmission line.
 4. The optical amplifying and relaying systemaccording to claim 3, wherein the wavelength selective reflecting meansis provided preceding to and subsequent to the variable opticalattenuator.
 5. The optical amplifying and relaying system according toclaim 3, wherein an optical isolator is provided as an intermediatestage in each monitoring light signal folding-back line, and eachwavelength selective reflecting means includes an optical fiber grating.6. The optical amplifying and relaying system according to claim 3,wherein the wavelength selective reflecting means is disposed at leastone of preceding to and subsequent to the variable optical attenuator.7. The optical amplifying and relaying system according to claim 3,wherein an optical isolator is provided as an intermediate stage in eachmonitoring light signal folding-back line.
 8. The optical amplifying andrelaying system according to claim 3, wherein each wavelength selectivereflecting means includes an optical fiber grating.
 9. The opticalamplifying and relaying system according to claim 1, wherein thewavelength selective reflecting means is provided preceding to andsubsequent to the variable optical attenuator.
 10. The opticalamplifying and relaying system according to claim 1, wherein an opticalisolator is provided as an intermediate stage in each monitoring lightsignal folding-back line, and each wavelength selective reflecting meansincludes an optical fiber grating.
 11. The optical amplifying andrelaying system according to claim 1, wherein wavelength selectivereflecting means is provided at least one of preceding to and subsequentto the variable optical attenuator.
 12. The optical amplifying andrelaying system according to claim 1, wherein an optical isolator isprovided as an intermediate stage in each monitoring light signalfolding-back line.
 13. The optical amplifying and relaying systemaccording to claim 1, wherein each wavelength selective reflecting meansincludes an optical fiber grating.
 14. The optical amplifying andrelaying system according to claim 2, wherein the variable opticalattenuators each supply a predetermined attenuation amount to themonitoring light signal based on a state of each optical amplifier. 15.The optical amplifying and relaying system according to claim 3, whereinthe variable optical attenuators each supply a predetermined attenuationamount to the monitoring light signal based on a state of each opticalamplifier.